For quantum communication systems, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time and are termed quantum signals. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
An example of quantum communication is quantum key distribution (QKD) which results in the sharing of cryptographic keys between two parties; a transmitter, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.